1. Field of the Disclosure
This disclosure relates to a failure detection technique in a communication system.
2. Discussion of the Background Art
A Passive Optical Network (PON) is an optical access network in a point-to-multipoint in which one or more optical subscriber line terminating devices (ONU: Optical Network Units) as user side transmission devices are connected to an optical subscriber line termination board (OSU: Optical Subscriber Unit) as a station-building side transmission device through an optical fiber and a splitter.
In the PON, PON protection is a technique of making an OSU, an ONU, an optical fiber, or a splitter as a constitutive factor of a system redundant and avoiding system down due to OSU failure, ONU failure, optical fiber breakage, and splitter breakage (non-patent document 1, pp. 113-116). Namely, upon applying the PON protection, when those constitutive factors of the system fail or are broken, operation is switched from a normal system to a redundancy system, whereby a service is continued.
To make OSU redundant is particularly considered important in the PON protection. As a first reason, since the OSU is an aggregation point of signals in point-to-multipoint communication, that is, PON, there is such a wide-range property of influence that when the OSU fails, all the subordinated ONUs (users) are affected. As a second reason, since the OSU is constituted of electronic components and software operating them, a possibility of a failure is highly likely to occur in comparison with an optical fiber/splitter as a mere optical waveguide. Accordingly, usually the OSU should be first made redundant in the PON protection.
One to a plurality of OSUs, an exchange switch (connected to the high-order side of the one to a plurality of OSUs), a high-order network interface (connected to the high-order side of the exchange switch), and an OLT controller controlling them are usually collectively stored in an optical subscriber line terminal station device (OLT: Optical Line Terminal). Namely, the OLT is a communication node exchanging the high-order network with PON, and a large number of OSUs which become interfaces on the PON side may be required. Accordingly, when the OSU is made redundant, an optimum constitution is required to be considered based on such a basic constitution of the OLT (in each drawing, the exchange switch, the high-order network interface, and input and output lines associated with them are omitted in terms of avoiding complexity).
In a first constitution in a conventional OSU redundant technique, as shown in FIG. 1, a redundancy system OSU 12 is provided for each normal system OSU 11 in an OLT 10 (Type-B of non-patent documents 1, p. 114). For convenience, the first constitution is referred to as a “redundancy system OSU independent type”. Specific configuration and operation of the “redundancy system OSU independent type” are as follows. A normal system OSU 11-1 and a redundancy system OSU 12-1 are connected to an ONU 51 (the number of the ONUs is an integer not less than 0, and ports on the ONU side of a splitter may include a free port) through an N:2 splitter 41-1 (N is an integer of not less than 1) (a splitter is further installed farther away than the N:2 splitter, and further branching may be performed). The normal system OSU 11-1 transfers a signal from the lower ONU 51 to the upper exchange switch and transfers a signal from the upper exchange switch to the lower ONU 51 respectively. When the normal system OSU 11-1 fails during communication, an OLT controller 15 having detected the failure performs switching,
(i) the normal system OSU 11-1 is stopped,
(ii) the redundancy system OSU 12-1 is started, and
(iii) setting of the exchange switch is changed.
Then, the redundancy system OSU 12-1 starts to transfer the signal from the lower ONU 51 to the upper exchange switch and transfer the signal from the upper exchange switch to the lower ONU 51 similarly to before the occurrence of the failure and thereby continues a service.
A connection form between a normal system OSU 11-i and a redundancy system OSU 12-i (i=2, 3, . . . ) and the operation at the time of protection are the same as those in the relation between the OSU 11-1 and the OSU 12-1.
In the constitution of the “redundancy system OSU independent type”, since the same number of the redundancy system OSUs and the normal system OSUs are required to be previously stored in the OLT, there are problems of OLT slot consumption and an increase in OSU introduction cost.
Meanwhile, in a second constitution of the conventional OSU redundant technique, as shown in FIG. 2, the redundancy system OSU 12 is provided in the OLT 10 with respect to a plurality of the normal system OSUs 11, whereby the problems of the OLT slot consumption and the increase in cost are suppressed (FIG. 2: FIG. 1 of patent document 1 and FIG. 1 of patent document 2). For convenience, the second constitution is referred to as a “redundancy system OSU sharing type”. Specific configuration and operation of the “redundancy system OSU sharing type” are as follows.
The OLT 10 stores a normal system OSU 11-i (i=1, 2, 3, . . . , and M, M is an integer of not less than 1) and a redundancy system OSU 12-x. Input and output ports on the ONU side of the redundancy system OSU 12-x is connected to an input and output port (referred to as a port x) of an M:1 optical switch 32. The M input and output ports (referred to as ports 1, 2, 3, . . . , and M) on the ONU side of the M:1 optical switch 32 are paired as the port 1 and the OSU 11-1, the port 2 and the OSU 11-2, the port 3 and the OSU 11-3, . . . , and the port M and the OSU 11-M, and these pairs are connected to N or less ONUs 50 through N:2 splitters 41-i (i=1, 2, 3, . . . , and M) (N ports on the ONU side of the splitter may include a free port).
The normal systems OSU 11 each transfer a signal from the lower ONU 50 to the upper exchange switch and transfer a signal from the upper exchange switch to the lower ONU 50. When the normal system OSU 11-1 fails during communication, the OLT controller 15 having detected the failure performs switching,
(i) the normal system OSU 1 is stopped,
(ii) the port x and the port 1 are connected in the optical switch 32,
(iii) a redundancy system OSUx is started, and
(iv) setting of the exchange switch is changed.
Then, the redundancy system OSU 12-x starts to transfer the signal from the lower ONU 50 to the upper exchange switch and transfer the signal from the upper exchange switch to the lower ONU 50 respectively similarly to before the occurrence of the failure and thereby continues a service.
It is the same as the above description that the operation of the failure of other normal system OSUs 11-i (i=2, 3, and . . . ), except that the normal system OSU 11, the splitter 41, and the port of the optical switch 32 are exchanged.
In this description (FIG. 2), the optical switch 32 is controlled by the optical switch controller 31, and moreover, the OLT controller 15 controls the optical switch controller 31. However, the optical switch controller 31 may not be provided, and the optical switch 32 may be controlled directly by the OLT controller 15.
In the constitution of the “redundancy system OSU sharing type”, since only the single redundancy system OSU 12 stored in the OLT 10 may be provided with respect to the M normal system OSUs 11, the problems of the OLT slot consumption and the increase in the OSU introduction cost have been solved. Further, under the condition that there is a sufficiently low probability that a plurality of the normal system OSUs fail simultaneously, it is easy to secure reliability by making the OSU redundant in this constitution and operation.
In the OLT, although the exchange switch, the high-order network interface, and the OLT controller may be made redundant, this case is not directly related in the disclosure, and therefore, description thereof will be omitted.